Week of April 20, 2008
I've finally received my SPI lap and another 50 carats of 3000 mesh
diamond powder so I can finish up flattening the top of the granite
straight edge. However, before I can begin, I need to clean and
check the lap to make sure it's flat and free from burrs. I blued
up the lap and set it on the surface plate. It appears that this
plate was ground pretty close to accurate, but it could still stand to
be broken in a bit before I use it on the straight edge.

I usually don't ask a seller to do
a extra nice job of packaging, but since this plate was 70 pounds, I
did on this occasion. This is what I got for my asking. The
cheap cardboard, coupled with the heavy handling that packages normally
get, left this box a bit the worse for the wear. Fortunately, the
shipping gods were on my side (for a change) and the plate arrived
without a scratch. I still don't know how I got that lucky.

I got out my diamond powder roller, which is nothing more than a roller
bearing that has been attached to an axle and bolted to an aluminum
handle. The procedure for charging then lap is to sprinkle some
diamond powder on one area of the lap and use the roller to press it
into the lap's surface. I tend to think that I get
better results if I use a lot of pressure on the roller, but with the
hardness of the diamonds, they will embed even if moderate pressure is
used. Using a larger roller would speed up the process some, but
my next step also helps embed more of the abrasives.

The next step was to empty the loose diamond powder from the lap on to
the top of the surface plate I have been reconditioning. Using a
coarse camel-hair artist's paint brush, I spread the powder out to form
a
square the size of the lap. The lap is then placed face-down on
the surface plate and pushed and twisted into the powder. At this
point the lap is lifted off the plate and the roller is used again to
press more diamond powder into the lap. The process is repeated
until the surface of the lap shows a uniform coat of the
abrasive. The diamond powder pressed into the lap shows as a dull
finish in contrast to the new shiny lap.

I wanted to try a test run on the old surface plate before I started
lapping the critical top surface of the straight edge. This would
give me an idea of how much granite will be removed in a few
passes. Generally one thinks of lapping metal as a surface
finishing technique that removes very little material. However
when lapping granite, my experience tells me that even with a rather
fine 3000 mesh powder, I can remove enough material to change the
surface topology substantially.

I have mapped the surface of the surface plate I am reconditioning by
drawing a grid of 2" squares that correspond to locations on the plate
and using my shop made planekator
copy I have record the relative height of each corner and center of
each
square. After looking at my map and finding the high points I
needed to lower, I tried out my new lap by rubbing it over the high
spots and feathering them into the areas that I needed to take less
material from. After a few passes, I removed the lap, cleaned the
plate and remeasured. After noting all of the relative heights on
the map, I found that I had been successful in lowering the highest
spots by a couple ten-thousandths. I had also achieved my other
goal of breaking in the lap. The lap's surface was now quite dull
where it had been in contact with the surface plate and less dull in
the lower areas of the lap. The pattern on the lap of dull and
less dull was pretty even and the area I had reworked on the surface
plate was smooth and uniform. I was ready to work on the granite
straight edge.

Since I didn't have much of a height difference between the highest and
lowest areas on the top of the straight edge, I chose to not recharge
the lap. I have read that by not having any loose diamond powder
on the surface and only using the diamonds embedded in the lap that one can
achieve a smoother surface with less variations in surface
elevation. Since this was what I was trying for, I gave it a shot.

With a ball bearing roller to
apply the 3000 mesh diamond powder, I rolled it in as evenly as I
could. I then did a practice run on my carefully mapped old
surface plate. I then reapplied more diamond powder and carefully
lapped the top of the granite straight edge. After checking level
in front and behind the straight edge, I checked the top surface of the
straight edge. After a couple cycles, it was level and smooth.

After a few passes on the top surface of the straight edge, I set up my
measuring gear so I could check the relative height of the top
surface. This turned out to be as close to zero difference in
height as I can measure. For the last check, I leveled my new
Starrett surface plate with the straight edge centered on it.
When I had the bubble centered on my 0.0002" per 10" box level while on
both sides of the straight edge, I placed the level on the top
surface of the straight edge. In this position, the level also
centered the bubble. This was no surprise since the Supramess
indicator that I used to measure the relative height of the top surface
has much better resolution (20 millionths) than the level, but I was
happy for the confirmation none-the-less.

So, one more mini-project is done and I now have a way to check level
between the separated ways of the surface grinder. This should
help me to make sure that both the saddle and table on the surface
grinder stay level and square with the vertical column. As a part
of the project, I have now acquired a lap that I will use only for 3000
mesh diamonds. The 3000 mesh diamonds leave a pretty nice finish
on granite, but it would be nice to have one more lap that I can devote
to 5000 mesh only for situations where I would prefer an even finer
finish.

Week of April 27, 2008
With the granite straight edge now finished, it was time to return to
scraping the two cast iron straight edges that will be used to confirm
that my granite straight edge's bottom surface is as flat as is needed
for functioning as a scraping master. The first one I started on about
3 weeks ago and it has progressed to the point of being almost done
with a pretty consistent 20 to 30+ bearing points per square inch when
spotted on the granite master. It is still not quite as smooth as
I would prefer but it's getting there.

The second cast iron straight edge progressed much more quickly by
using a pull-type scraper. As I see it, there are a couple
advantages and a couple disadvantages to using a pull scraper. On
the plus side, I have more control pulling than I do pushing. For
me, this means that it is easier to stop a pull stroke than a push
stroke. This results in each scrape being only as long as the
high spot I am trying to remove rather than extending past the high
spot into the surrounding area. Another advantage is the depth of
cut. When using a pull stroke, it is easier for me to keep the
depth of the cut constant than it is when pushing. When I have
progressed to taking shallow cuts, I am less likely to gouge the
surface when pulling. On the flip side, it is more difficult for
me to work close to edges with a pull stroke. Pushing the scraper
off an edge at a 45° angle while holding the edge of the scraper
parallel to the surface of the plane being scraped can be mastered
fairly quickly. Pulling the blade on to an edge at a 45°
angle with only a small amount of the blade actually in contact with
the surface being scraped and trying to keep the blade parallel to the
scraped surface is much more difficult for me to master. The
tendency is to gouge the edge as the scraper is pulled on to the
surface. One answer is to only pull the blade off of the edge
rather than on to the edge, but there are times when this isn't
possible. The other disadvantage of the pull scraper that comes
to mind is trying to use it for scraping near the vertex of an inside
dovetail. A push scraper with its thin blade will fit much deeper
than the thicker profile of a pull scraper.

Disadvantages aside, pull scraping the cast iron straight edge that I
can move around to get the best angle of attack was the quickest that I
have ever scraped from rough to almost surface plate quality
finish. It took only about 4 days at 3 hours work a day. I
was simply amazed at how quickly the surface improved. Since my
main reason for scraping the second c.i. straight edge was to test it
against the other cast iron straight edge and prove the flatness of my
granite master, I was checking it against the other straight edge as I
scraped each cycle.
With the two c.i. straight edges spotting each other, it was looking
very good. The pattern of high spots I received while spotting
against the almost finished cast iron were very similar to the
patterns I received when spotting against the granite master.. By the
end of the forth day, the pictures below were taken. If you click
to enlarge them, you'll see how well the prints match. The prints
are a bit exaggerated by the use of more Prussian blue than necessary,
but that is the only way the camera picks up the patterns. When I
spot with the bare minimum of blue, the color transferred to the clean
straight edge is even and shows that the two cast iron straight edges
fit together almost as well as each cast iron fits to the granite
master. Unfortunately, I am not able to produce a good picture of
the very faint markings.

Left: Cast iron straight edges
spotted on each other. Center: Closer view. Right: A nice
pattern showing that the two cast iron straight edges print as well on
each other as they do on the granite master straight edge. The
one on the left (first pic), bottom (2nd), left (3rd) was pull-scraped
and took about a third the time to do.

.
May 04, 2008
This past week and the next week are going to be pretty slow on
progress in the shop. Our son graduates from the Army's Ranger school
and we'll be in Georgia at Fort Benning for a couple days.
Congratulations Michael! It's also time to do the spring
maintenance on the vehicles, yard and whatever other around the house
projects need to be done. Hopefully these won't take too long as
I would like to get beck to scraping the inverted V ways on the
grinder's table. It is also getting close to time to check my
progress on the table against the grinder. This will involve some
leveling and a lot of measurements. The timing for this project
pretty good as I am just finishing up on reading Fundamentals of Dimensional
Metrology
by Ted Busch and will
be trying to keep a lot of what I've learned in mind.

May 11, 2008
Back to work on the surface grinder. I'm about ready to start
another mini-project to help me keep the table ways in alignment with
each other. I have been wanting to buy a King-Way alignment
system for a while now. Unfortunately they don't come up on Ebay
very often and when they do, even the ratty ones with missing parts
seem to sell in the $300.00+ range. With the prospect of getting
one at anywhere near a price I can afford is not too good, I have
decided to build one. There's a patent drawing of the tool here
that will give you an idea of how it looks and works. It is
really a pretty simple concept. There are two upright posts that
attach to a cross-bar. On the cross-bar there is a two axis
spirit level. On the bottom of one upright, there is a heavy
walled tube with a slot cut lengthwise. This slot will ride on an
inverted V way or the sides of the tube would ride between the sides of
a non-inverted V way. On the bottom of the other upright, there
is a steel ball that rides on a flat way. A steel ring of a
slightly smaller diameter can be placed under the ball if the flat ways
are too dinged up to rely on the single contact point of the
ball. By leveling the tool, then sliding it along the ways, one
can see if the ways remain parallel to each other by observing the
level's bubble. Since my smallest precision level (6") has only one
vial, I will only be able to measure one axis at a time, but this does
not diminish the usefulness of the tool. I will be on the lookout
for a two axis, 0.0005" discrimination spirit level to take the place
of my single axis one.

I have already made a mock-up of the tool using two dial indicator
holders with magnetic bases, my 6" level, and some odds and ends and
even this crude setup is a great help in keeping the inverted V ways of
the surface grinder table parallel with the flat way that has already
been scraped flat. Building a copy of the King-Way will reduce
the number of parts in comparison to my mock-up and less parts equals
less chance of alignment errors. Once I've gathered all of the
pieces I'll need to build the tool, it should be a pretty quick
project. If you're interested in reading a bit more about the
King-Way alignment tool and taking a look at the original instructions
that came with the tool, there's a thread
at Practical Machinist that's worth reading.